Check it out! When the wheel isn't spinning, it's easy to tilt it from side
to side. But when the wheel is given a spin, its forward momentum resists
side to side movement. Try it yourself. You'll need a friend or parent to
spin the wheel, and be careful to keep your fingers away from the spokes.

The Gyroscope and Direction Control
If you've ever seen a toy gyroscope, you know that it's a device that has a
spinning mass, usually a disk or wheel, attached to a base that allows the axis
of the spinning mass to move freely. Like a child's top, the toy gyroscope can
"stand" on its tip. The reason is that the spinning mass (of the gyroscope and
the top) resists a change in motion. (If you don't have firsthand experience
with a gyroscope, try this: Hold the ends of a bicycle wheel's axle in each
hand while you have someone spin the wheel, then try to tilt the wheel back and
forth.)

The WWII torpedo takes advantage of the gyroscope's resistance to a change in
motion. It uses this property to allow a course to be "programmed" into the
torpedo.

The axis of this torpedo's gyroscope points down the length of the torpedo. To
set the gyroscope, a ring that surrounds the gyroscope is moved (either
automatically by the submarine's computer or manually by the torpedoman) while
the torpedo is still on board. The position that the ring is set to depends on
the heading that the torpedo must take once it leaves the submarine.

Firing the torpedo opens a valve that causes pressurized air from the air
chamber to spin the gyroscope. As the torpedo leaves the submarine, the
gyroscope is still lined up with the submarine and with the torpedo itself; the
spinning mass will keep it pointing in the same direction, even when the
torpedo changes its heading. After traveling a set distance (known as the
reach), the gyro mechanism kicks in. It senses that the ring and the axis of
the gyroscope are not aligned. Via a connection to the torpedo's two
directional rudders, it begins to turn the torpedo in the correct direction.

Once the ring and the gyro axis line up, the rudders straighten out. From this
point on, the gyro mechanism makes minor steering adjustments via the rudders,
keeping the torpedo on a straight course.

The Pendulum and Depth Control
Perhaps when you hear the word pendulum you think of a clock's swinging
pendulum and how, when the clock is working, it's always in motion, moving back
and forth at a constant rate. The pendulum in a torpedo is not unlike the one
in a clock, with one crucial exception: The torpedo's pendulum tries to stay
stationary.

The torpedo's pendulum is connected to the two rudders that control depth. If
the torpedo, while it's in motion, begins to point down, the pendulum rocks
forward relative to the torpedo, causing the rudders to guide the torpedo up.
If the torpedo begins to point up, the pendulum rocks back. This causes the
rudders to steer the torpedo down.

So the pendulum's job is to keep the torpedo level. The torpedo's depth is
controlled by a diaphragm and compressed spring, which senses the pressure of
the water outside the torpedo and adjusts the rudders accordingly.